A simple continuous-flow chemiluminometric method for the determination of 0.0540-5.40 mug/ml of ammonium ion is described. The method is based on the chemiluminescence generated during the oxidation of ammonium ion by N-bromosuccinimide in alkaline medium. The emission intensity is greatly enhanced if dichlorofluorescein is also present in the reaction solution. The analysis is automated, requires no sample pre-treatment and solutions can be analysed at a rate of 106 solutions/hr with a relative error of about 2.5%. The method was applied satisfactorily to the determination of ammonium ion in solid and liquid fertilizers.